1. Field of the Invention
The present invention relates to a process for producing an aromatic carbonate. More particularly, the present invention is concerned with a process for producing an aromatic carbonate which comprises transesterifying, in the presence of a metal-containing catalyst, a starting material selected from the group consisting of a dialkyl carbonate, an alkyl aryl carbonate and a mixture thereof with a reactant selected from the group consisting of an aromatic monohydroxy compound, an alkyl aryl carbonate and a mixture thereof, wherein the transesterification is conducted while maintaining a weight ratio (WR) of at least one aromatic group-containing substance selected from the group consisting of a specific aromatic polyhydroxy compound and a residue thereof to the metal of the metal-containing catalyst at 2.0 or less, wherein the weight ratio (WR) is measured with respect to a catalyst-containing liquid-phase mixture in a system for the transesterification, and wherein the aromatic group-containing substance originates from the starting material, the reactant and/or a by-product of the transesterification. According to the process of the present invention, the transesterification reaction can be conducted without suffering disadvantageous phenomena, such as the deposition of the catalyst from the catalyst-containing liquid-phase mixture and the adhesion of the deposited catalyst to the inner walls of the reactor, pipes and the like, so that the desired aromatic carbonate can be produced stably for a prolonged period of time.
2. Prior Art
An aromatic carbonate is useful as a raw material for, e.g., the production of an aromatic polycarbonate (whose utility as engineering plastics has been increasing in recent years) without using poisonous phosgene. With respect to the method for the production of an aromatic carbonate, a method for producing an aromatic carbonate or an aromatic carbonate mixture is known, in which a dialkyl carbonate, an alkyl aryl carbonate or a mixture thereof is used as a starting material and an aromatic monohydroxy compound, an alkyl aryl carbonate or a mixture thereof is used as a reactant, and in which a transesterification reaction is performed between the starting material and the reactant.
However, since this type of transesterification is a reversible reaction in which, moreover, not only is the equilibrium biased toward the original system but the reaction rate is also low, the production of an aromatic carbonate by the above-mentioned method on an industrial scale is accompanied with great difficulties.
To improve the above-mentioned method, several proposals have been made, most of which relate to the development of a catalyst for increasing the reaction rate. As a catalyst for use in the method for producing an alkyl aryl carbonate, a diaryl carbonate or a mixture thereof by reacting a dialkyl carbonate with an aromatic hydroxy compound, there have been proposed various metal-containing catalysts, which include for example, a Lewis acid, such as a transition metal halide, or compounds capable of forming a Lewis acid, see Unexamined Japanese Patent Application Laid-Open Specification No. 51-105032, Unexamined Japanese Patent Application Laid-Open Specification No. 56-123948 and Unexamined Japanese Patent Application Laid-Open Specification No. 56-123949 (corresponding to West German Patent Application Publication No. 2528412, British Patent No. 1499530 and U.S. Pat. No. 4,182,726)!, a tin compound, such as an organotin alkoxide or an organotin oxide Unexamined Japanese Patent Application Laid-Open Specification No. 54-48733 (corresponding to West German Patent Application Publication No. 2736062), Unexamined Japanese Patent Application Laid-Open Specification No. 54-63023, Unexamined Japanese Patent Application Laid-Open Specification No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110), Unexamined Japanese Patent Application Laid-Open Specification No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704), Unexamined Japanese Patent Application Laid-Open Specification No. 62-277345, and Unexamined Japanese Patent Application Laid-Open Specification No. 1-265063!, salts and alkoxides of an alkali metal or an alkaline earth metal (Unexamined Japanese Patent Application Laid-Open Specification No. 56-25138), lead compounds (Unexamined Japanese Patent Application Laid-Open Specification No. 57-176932), complexes of a metal, such as copper, iron or zirconium (Unexamined Japanese Patent Application Laid-Open Specification No. 57-183745), titanic acid esters Unexamined Japanese Patent Application Laid-Open Specification No. 58-185536 (corresponding to U.S. Pat. No. 4,410,464)!, a mixture of a Lewis acid and protonic acid Unexamined Japanese Patent Application Laid-Open Specification No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501)!, a compound of Sc, Mo, Mn, Bi, Te or the like Unexamined Japanese Patent Application Laid-Open Specification No. 1-265064 (corresponding to European Patent Publication No. 0 338 760 A1 and U.S. Pat. No. 5,034,557)!, and ferric acetate (Unexamined Japanese Patent Application Laid-Open Specification No. 61-172852).
As a catalyst for use in the method for producing a diaryl carbonate by a same-species intermolecular transesterification, wherein an alkyl aryl carbonate is disproportionated to a diaryl carbonate and a dialkyl carbonate, there have been proposed various catalysts, which include for example, a Lewis acid and a transition metal compound which is capable of forming a Lewis acid see Unexamined Japanese Patent Application Laid-Open Specification No. 51-75044 (corresponding to West German Patent Application Publication No. 2552907 and U.S. Pat. No. 4,045,464)!, a polymeric tin compound Unexamined Japanese Patent Application Laid-Open Specification No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110 and West German Patent Application Publication No. 3445552)!, a compound represented by the formula R--X(.dbd.O)OH (wherein X is selected from Sn and Ti, and R is selected from monovalent hydrocarbon residues) Unexamined Japanese Patent Application Laid-Open Specification No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704)!, a mixture of a Lewis acid and protonic acid Unexamined Japanese Patent Application Laid-Open Specification No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501)!, a lead catalyst (Unexamined Japanese Patent Application Laid-Open Specification No. 1-93560), a titanium or zirconium compound Unexamined Japanese Patent Application Laid-Open Specification No. 1-265062 (corresponding to U.S. Pat. No. 5,166,393)!, a tin compound Unexamined Japanese Patent Application Laid-Open Specification No. 1-265063 (corresponding to European Patent Publication No. 0 338 760 A1 and U.S. Pat. No. 5,034,557)!, and a compound of Sc, Mo, Mn, Bi, Te or the like Unexamined Japanese Patent Application Laid-Open Specification No. 1-265064 (corresponding to European Patent Publication No. 0 338 760 A1 and U.S. Pat. No. 5,034,557)!.
Another attempt for improving the yield of aromatic carbonates in these reactions consists in biasing the equilibrium toward the product system as much as possible, by modifying the mode of the reaction process. For example, there have been proposed a method in which by-produced methanol is distilled off together with an azeotrope forming agent by azeotropic distillation in the reaction of a dimethyl carbonate with phenol see Unexamined Japanese Patent Application Laid-Open Specification No. 54-48732 (corresponding to West German Patent Application Publication No. 2736063 and U.S. Pat. No. 4,252,737) and Unexamined Japanese Patent Application Laid-Open Specification No. 61-291545!, and a method in which by-produced methanol is removed by adsorbing the same onto a molecular sieve Unexamined Japanese Patent Application Laid-Open Specification No. 58-185536 (corresponding to U.S. Pat. No. 4,410,464)!.
Further, a method is known in which an apparatus comprising a reactor having provided on the top thereof a distillation column is employed in order to separate and distill off alcohols (by-produced in the course of the reaction) from a reaction mixture obtained in the reactor. With respect to this method, reference can be made to, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 56-123948 (corresponding to U.S. Pat. No. 4,182,726), Unexamined Japanese Patent Application Laid-Open Specification No. 56-25138, Unexamined Japanese Patent Application Laid-Open Specification No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110), Unexamined Japanese Patent Application Laid-Open Specification No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704), Unexamined Japanese Patent Application Laid-Open Specification No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501), Unexamined Japanese Patent Application Laid-Open Specification No. 61-172852, Unexamined Japanese Patent Application Laid-Open Specification No. 61-291545, and Unexamined Japanese Patent Application Laid-Open Specification No. 62-277345.!
As more preferred methods for producing an aromatic carbonate, the present inventors previously developed a method in which a dialkyl carbonate and an aromatic hydroxy compound are continuously fed to a continuous multi-stage distillation column to effect a continuous transesterification reaction in the distillation column, while continuously withdrawing a low boiling point reaction mixture containing a by-produced alcohol from an upper portion of the distillation column by distillation and continuously withdrawing a high boiling point reaction mixture containing a produced alkyl aryl carbonate from a lower portion of the distillation column see Unexamined Japanese Patent Application Laid-Open Specification No. 3-291257 (corresponding to U.S. Pat. No. 5,210,268 and European Patent Publication No. 0 461 274 B1)!, and a method in which an alkyl aryl carbonate is continuously fed to a continuous multi-stage distillation column to effect a continuous transesterification reaction in the distillation column, while continuously withdrawing a low boiling point reaction mixture containing a byproduced dialkyl carbonate from an upper portion of the distillation column by distillation and continuously withdrawing a high boiling point reaction mixture containing a produced diaryl carbonate from a lower portion of the distillation column see Unexamined Japanese Patent Application Laid-Open Specification No. 4-9358 (corresponding to U.S. Pat. No. 5,210,268 and European Patent Publication No. 0 461 274 B1)!. These methods for the first time realized efficient, continuous production of an aromatic carbonate. Thereafter, various methods for continuously producing an aromatic carbonate have further been developed, based on the above-mentioned methods developed by the present inventors. Examples of these methods include a method in which a catalytic transesterification reaction is performed in a column reactor see Unexamined Japanese Patent Application Laid-Open Specification No. 6-41022 (corresponding to West German Patent Application Publication No. 4218061, European Patent Publication No. 0 572 870 A1 and U.S. Pat. No. 5,362,901), Unexamined Japanese Patent Application Laid-Open Specification No. 6-157424 (corresponding to West German Patent Application Publication No. 4226755, European Patent Publication No. 0 582 931 A1 and U.S. Pat. No. 5,334,742), Unexamined Japanese Patent Application Laid-Open Specification No. 6-184058 (corresponding to West German Patent Application Publication No. 4226756, European Patent Publication No. 0 582 930 A1 and U.S. Pat. No. 5,344,954)!, a method in which use is made of a plurality of reactors which are connected in series Unexamined Japanese Patent Application Laid-Open Specification No. 6-234707 (corresponding to West German Patent Application Publication No. 4301899, European Patent Publication No. 0 608 710 A1 and U.S. Pat. No. 5,463,102), and Unexamined Japanese Patent Application Laid-Open Specification No. 6-263694!, a method in which a bubble tower reactor is used Unexamined Japanese Patent Application Laid-Open Specification No. 6-298700 (corresponding to West German Patent Application Publication No. 4316428, European Patent Publication No. 0 614 877 A1 and U.S. Pat. No. 5,523,451)!, and a method in which a vertically long reactor vessel is used (Unexamined Japanese Patent Application Laid-Open Specification No. 6-345697).
Also, there have been proposed methods for producing an aromatic carbonate stably for a prolonged period of time on a commercial scale. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 6-157410 (corresponding to European Patent Publication No. 0 591 923 A1 and U.S. Pat. No. 5,380,908) discloses a method for producing aromatic carbonates from a dialkyl carbonate and an aromatic hydroxy compound, which comprises continuously supplying a mixture of raw materials and a catalyst to a reactor provided with a distillation column thereon to effect a transesterification reaction in the reactor, while continuously withdrawing a by-produced aliphatic alcohol from the reactor through the distillation column by distillation so as to keep the aliphatic alcohol concentration of the reaction system at 2% by weight or less. This prior art document describes that, by this method, continuous production of an aromatic carbonate can be performed in a stable manner. The object of this method is to avoid the deposition of the catalyst in the distillation column. However, the reason for the deposition of the catalyst in the distillation column to be able to be avoided by this method is very simple and resides in that the catalyst is not fed to the distillation column. This prior art document describes the effect that since the catalyst was fed only to the reactor, no clogging was observed in the distillation column. This document also describes that, when a catalyst is fed to the distillation column, clogging of a part of the distillation column occurs due to the deposition of the catalyst in the column. In view of the fact that, in this method, a catalyst is not fed to the distillation column, it is natural and not surprising that the deposition of the catalyst does not occur in the distillation column. In addition, in view of the fact that, in this method, a catalyst is not fed to the distillation column, it is also apparent that, by this method, it is impossible to efficiently conduct a reaction by feeding a catalyst to a distillation column (that is, it is impossible to conduct a reactive distillation by this method). Further, this prior art document does not teach or suggest any solution for the problem where a catalyst is deposited from a catalyst-containing liquid-phase mixture and the deposited catalyst adheres to a reactor, pipes, valves, means for separating the catalyst from the liquid phase, and the like in the system for transesterification. In this prior art document, the reason why the aliphatic alcohol concentration of the reaction system should be kept at 2% by weight or less is not explained. However, it is presumed that the aliphatic alcohol concentration requirement of 2% by weight or less is intended to bias the equilibrium toward the product system so that the reaction proceeds efficiently. In this prior art document, there is no description regarding the relationship between the aliphatic alcohol concentration and the deposition of a catalyst.
Unexamined Japanese Patent Application Laid-Open Specification No. 6-116210 (corresponding to European Patent Publication No. 0 591 923 A1 and U.S. Pat. No. 5,380,908) discloses a method for producing a diaryl carbonate from an alkyl aryl carbonate, which comprises continuously supplying a mixture of raw materials and a catalyst to a reactor provided with a distillation column thereon to effect a transesterification reaction in the reactor, while continuously withdrawing a by-produced dialkyl carbonate from the reactor through the distillation column by distillation so as to keep the dialkyl carbonate concentration of the reaction system at 2% by weight or less. This prior art document describes that, by this method, continuous production of a diaryl carbonate can be performed in a stable manner. The object of this method is to avoid the deposition of the catalyst in the distillation column. However, the reason why the deposition of the catalyst in the distillation column is able to be avoided by this method is very simple and resides in that the catalyst is not fed to the distillation column. This prior art document describes the effect that since the catalyst was fed only to the reactor, no clogging was observed in the distillation column. This document also describes that, when a catalyst is fed to the distillation column, clogging of a part of the distillation column occurs due to the deposition of the catalyst in the column. In view of the fact that, in this method, a catalyst is not fed to the distillation column, it is natural and not surprising that the deposition of the catalyst does not occur in the distillation column. In addition, in view of the fact that, in this method, a catalyst is not fed to the distillation column, it is also apparent that, by this method, it is impossible to efficiently conduct a reaction by feeding a catalyst to a distillation column (that is, it is impossible to conduct a reactive distillation by this method). Further, this prior art document does not teach or suggest any solution for the problem where a catalyst is deposited from a catalyst-containing liquid-phase mixture and the deposited catalyst adheres to a reactor, pipes, valves, means for separating the catalyst from the liquid phase, and the like in the system for transesterification.
When a catalyst is deposited from a catalyst-containing liquid-phase mixture and the deposited catalyst adheres to the inner walls of a reactor, pipes and the like, the reaction is adversely affected. That is, the deposited catalyst which is adhered to the inner walls of the reactor, pipes and the like cannot exhibit effective catalyst activity, even if it is present in the reactor. The deposition of the catalyst and the adhesion of the deposited catalyst to portions in the system for the transesterification means that the amount of the effective catalyst present in the reactor is decreased, so that the reaction cannot achieve favorable results.